Due to the features such as high liquid yield, high hydrogen yield and high aromatics yield and the like, the continuous regenerative catalytic reforming of naphtha drew extensive attention during the production of high-octane gasoline and aromatics. At present, the reforming catalysts used in the continuous reforming apparatus are a series of dual or multi-metal catalysts containing platinum-tin, and the platinum-tin catalyst is sensitive to sulfide as compared with the catalyst containing only platinum. Thus, to ensure the normal operation of the catalytic reforming units, the sulfur amount in the reforming feedstock should be strictly limited.
CN1234455C, U.S. Pat. No. 6,495,487B1 and U.S. Pat. No. 6,780,814B2 all disclose the requirements on the operating environment of a platinum-tin multi-metal reforming catalyst, and state that, during the normal operation of the continuous reforming reaction, the naphtha feedstock used for reforming is desulfurized via catalytic desulfurization and adsorption desulfurization to the minimum, and sulfur-free is optimal.
Petroleum Processing and Petrochemicals and Industrial Catalysis respectively introduce at pages 26-29, Vol. 33, No. 8, 2002 and at pages 5-8, Vol. 11, No. 9, 2003 the index requirements on controlling the impurity content of the reforming materials by using the platinum-tin series reforming catalyst while the continuous reforming is normally operated, wherein the sulfur amount is generally controlled to be not greater than 0.5 μg/g.
The continuous reforming has a relatively low operating pressure, a relatively high reaction temperature and a relatively low hydrogen/feedstock oil ratio, and the apparatus is easy to coke during the reaction. With the progress of the technology, the continuous reforming continuously develops in the direction of higher severity level, such as ultralow pressure, low hydrogen/feedstock oil ratio, low space velocity and the like, and the coking tendencies of the reactor and heating furnace tube also increase. Up to the present, the reactor walls of many sets of the continuous reforming apparatus have been coked. Coking will result in poor catalyst flow, impairment of the components in the reactor, or even shutdown of the apparatus, so as to do enormous economic losses to the refineries.
Catalytic Reforming Process and Engineering (1st Edition, 2006-11, China Petrochemical Press, p 522-534) analyzes the coking mechanism of the continuous reforming apparatus. In the reducing atmosphere, hydrocarbon molecules are adsorbed on the surface of the metal crystal grains of the reactor walls, and excessively dehydrogenated under the metal catalysis of the reactor walls to produce carbon atoms so as to dissolve into or penetrate into crystal grain or particle interstices. Due to charcoal deposition and growth, metal crystal grains are separated from the matrix, so as to produce fibrous carbon with iron particles at the top thereof. Such charcoal is notably different from the carbon deposit on the catalyst in that such charcoal has higher catalytic dehydrogenation and hydrogenolysis activities; the reaction continues at a high temperature as soon as it is produced; the generation rate continues to speed up, and the fibrous carbon continuously get longer, coarser and harder. The development of fibrous carbon generally undergoes several phases comprising soft carbon, soft bottom carbon and hard carbon. The longer the time for the formation thereof is, the more serious the consequences are. The initial stage of the coke formation in the apparatus may result in the blockage of the circulating system so that the normal circulation cannot be carried out. The severe coke formation will impair the inner components of the reactors, such as sectorial tube, central tube and the like. If the formed coke goes into the regeneration system, topical overtemperature of the charring zone in the regenerator and of the oxychlorination zone occur so as to burn out the inner components in the regenerator. The impairment of the inner components in the reactor and regenerator becomes more severe with the prolongation of the operation time.
In order to prevent the metal walls of the continuous reforming apparatus from catalytic coking, Catalytic Reforming (1st Edition, 2004-4, China Petrochemical Press, p 200-202) introduces that the currently well-known process comprises feeding organic sulfides into the reforming feedstocks during the normal reforming operation, controlling the sulfur amount of the reforming feedstocks to be 0.2-0.3 μg/g so as to inhibit the catalytic activity of the metal surfaces of the inner walls of the reactor and the heating furnace tube. However, Catalytic Reforming does not introduce feeding sulfides into the feedstocks when the feedstock oil is fed into the continuous reforming apparatus at a low temperature. A general option could involve feeding sulfides into the reaction system when the inlet of each reactor reaches to a temperature greater than 480-490° C.
Currently, on the basis of the requirements on the material balance, the hydrogen balance and the product of refineries, the continuous reforming operation will rapidly increase the reaction severity level after the feedstock oil is fed and when water in the gas is qualified. The sulfur amount in the reforming feedstock is controlled to be 0.2-0.5 μg/g. In particular, the newly-built apparatus firstly used is not sufficient to rapidly or adequately passivate the reactor walls and the heating furnace tube walls. After the above-mentioned passivation process is used in a significant part of the continuous reforming apparatus, coking of the reaction system still occurs during the operation. It thus becomes an important problem paid more attention to by the continuous reforming technician how to effectively inhibit the metal-catalyzed coking of the continuous reforming reactor walls and the heating furnace tube walls.
There are many processes for preventing hydrocarbons from coking at the high-temperature positions of the reactor in other fields of the petrochemical industry. CN1160435C discloses a method of inhibiting coke deposition in pyrolysis furnaces, comprising, before feeding the hydrocarbon feedstocks into the pyrolysis furnace, treating the pyrolysis furnace with a combination of sulfur- and phosphorus-containing compounds having a total sulfur to phosphorus atomic ratio of at least 5, adding a sufficient amount of sulfur-containing compounds into phosphorus-containing compounds so as to form a uniform and effective passivation layer on the surface of pyrolysis furnaces, thereby effectively inhibiting the coke deposition.
CN85106828A discloses a process for forming sulfide layer on the surface of metal parts and apparatus therefor, comprising laying the metal parts on the cathodic disk in the reaction chamber of the vacuum furnace, laying solid sulfur in the vacuum furnace, solid sulfur being vaporized by heating, gaseous sulfur bombarding the metal parts laid on the cathodic disk under the influence of an electric field to form sulfide layer on the surface thereof.
CN1126607C discloses a process for suppressing and relaxing generation and deposition of coke in high-temperature cracking of hydrocarbons, wherein, prior to feeding the cracking feedstocks, a pre-treating agent which is a mixture of one or several chosen from hydrogen sulfide, organosulfur compound, organophosphorus compound and organothiophosphorus compound, together with the water vapour are fed into the cracking apparatus to pre-treat the metal surface. Said process can passivate the metal surface of the cracking furnace so as to suppress and relax generation and deposition of coke during the cracking and subsequent treatment.
Since platinum-tin series continuous reforming catalysts are extremely sensitive to impurities and have high requirements on the environment, various substances involved in said processes all result in severe or irreversible poisoning of the reforming catalyst, thereby being not suitable for the catalytic reforming process.